1. Field of the Invention:
The invention relates to the field of material storage and metering systems and more particularly to the field of accurately metering and feeding small quantities of solid materials, especially, although not limited to, abrasive particulate material.
2. Prior Art:
Prior art systems for delivering particulate material include ones employing a discharge auger within an output tube in the lower portion of a storage bin, a type of system which is inadequate for accurately metering some materials. For some of these materials, the problem of accurate metering is overcome by dual auger systems having a feed auger which surrounds the portion of the discharge auger which is within the bin. The feed augers in these systems comprise a helical spiral which feeds excess material to the discharge auger to minimize air pockets and other density variations in the material being delivered. Unfortunately, neither of these types of auger systems are satisfactory for delivering highly abrasive materials because these materials scour the augers and the exit tubes through which the material passes and may even jam the systems. Even if the systems do not jam, the scouring reduces the life of the equipment and adds impurities to the material being fed.
Accurate delivery of pure aluminum oxide (Al.sub.2 O.sub.3) (which is abrasive) is required for the growth of single crystalline sapphire from a melt in a continuous process such as edge defined film fed growth (EFG). Very high purity material is required in order to grow high quality single crystalline sapphire. In order to maintain uniform growth conditions and maximize machine efficiency, the consumed source material in the melt must be replenished on a continuing basis during the growth of the sapphire.
There are several prior art systems designed specifically for delivering abrasive particles such as aluminum oxide. One of these systems comprises a disc having a major surface in a horizontal plane supported on a vertical shaft for rotating the disc within the horizontal plane. Material is deposited from an overhead hopper onto the upper surface of the rotating disc in order to cause the particles to traverse the disc surface in a controlled manner and fall off the edges of the disc into an exit chute. A system of this type is disclosed in U.S. Pat. No. 3,960,503.
Another one of these systems comprises a waterwheel-like thick circular disc having shallow cups or buckets in the periphery thereof for transporting small quantities of material as the disc is rotated about on a horizontal axis through the center of the vertically oriented disc.
A more accurate, more reliable, smoother metering system is needed in order to maintain the sapphire growth conditions substantially constant.